I have an 18' garage door. The header is two 2x12's with an 11" x 1/4" steel plate laminated between using 1/2" bolts to clamp it together. The steel plate is even with the 2x12's at the bottom. Condensation has caused the bottom of the outer 2x12 to rot. So I replace the outer 2x12, how do I insulate it to prevent this from happening again? Bottom of outer 2x12 has 1x8 treated below it forming the upper trim at the outside. In front of the outer 2x12 is 1/2" outer house sheathing and brick molding.

That steel,is called a flitch plate. Sorry I don't have an answer.

I will take a crack at this.
So you have one of 3 problems.
1. bulk water is getting at the outer header from above or wind blown rain from bad flashing/ water management details. Pictures might help us understand.
2. You are getting air moving through the assembly and the steel beam is a cold condensing surface so the moisture in the air is condensing at the wood beam interface. To stop this you need to prevent air from moving through the assembly. I could see this affecting the outer part of the header because the steel prevents inward drying, and the other 2x12 can dry out after it gets wet, preventing damage
3. You have moisture migration through the brick, and you only have a single layer of housewrap behind the brick so no drainage plane. The moisture moves through the brick, into the wood then is stopped by the steel which is an excellent water vapor barrier. Likely the sheathing would be rotted in the same area in this case Does this area of brick get wet from rain? Is it in direct sunlight? If so that may be your problem. This effect is called solar driven vapor drive, and is a big problem with fake stone and brickmold. It may be compounded by the fact that any moisture that gets in drains down to the horizontal 1x8 and then sits there with no where to go.

Hard to tell what is really going on without looking at it, but that is what it looks like from my house. Unfortunately the solutions depend on the problem, so I hesitate to recommend a particular course of action.  Add pictures and we can make better suggestions. 
Cheers,
Eric

Another alternative: replace the steel plate and the outside 2x12 with a laminated LVL beam. They are 1-3/4" thick. Height will be 11-7/8 but can be ripped down to fit, or doubled up if need be. They're available in 10,12,14,16" etc heights, any length. If you have the room and height on the inside of the garage, a higher (14, 16, 18") beam has greatly increased carrying capacity.